偶然接触一个python的关于状态机的库，简单demo如下：# coding=utf-8
##############################################################
# 目标："solid", "liquid", "gas", "plasma" 四种互相转换...
# 环境： pip install transitions
# 这个库简直碉堡了...
##############################################################
from transitions import Machine
class Matter(object):
pass
model = Matter()
# 定义状态
states = ["solid", "liquid", "gas", "plasma"]
# 定义状态转移
transitions = [
{'trigger':'melt', 'source': 'solid', 'dest': 'liquid'},
{'trigger':'evaporate', 'source': 'solid', 'dest': 'gas'},
{'trigger':'sublimate', 'source': 'liquid', 'dest': 'gas'},
{'trigger':'ionize', 'source': 'gas', 'dest': 'plasma'},
]
# 初始化
machine = Machine(model=model, states=states, transitions=transitions, initial='solid')
print model.state
model.melt()
print model.state
对源代码一探究竟。发现看不懂的地方甚多，那就分解一步步去解析吧。
Machine
—————> States = {'s1':s1, 's2':s2, 's3': s3}
—————> Events = {e1, e2} e1—————>Transition = [t1,t2,t3...]
—————> models = [m1, m2] m1 —————> to_s1(), to_s2()
大概就是Machine这个类里面维护比较重要的三个元素，States={}，Events={}, models= []，接下来先看怎么填充这3个元素。
0、pip install transitions==0.6.1
python == 2.7.10
1、
按照demo来说，我们传入的States和transitions：
States = ["solid", "liquid", "gas", "plasma"]
transitions = [
{'trigger':'melt', 'source': 'solid', 'dest': 'liquid'},
{'trigger':'evaporate', 'source': 'solid', 'dest': 'gas'},
{'trigger':'sublimate', 'source': 'liquid', 'dest': 'gas'},
{'trigger':'ionize', 'source': 'gas', 'dest': 'plasma'},
]2、
States
Machine的构造函数中，传入了一个状态list，触发self.add_states(states)：
if states is not None:
self.add_states(states) for state in states:
if isinstance(state, string_types):
state = self._create_state(state, on_enter=on_enter, on_exit=on_exit,
ignore_invalid_triggers=ignore, **kwargs)
self.states[state.name] = stateself.states = {
'solid': <State('solid')@4516320272>,
'liquid': <State('liquid')@4516320336>,
'gas': <State('gas')@4516320400>,
'plasma': <State('plasma')@4516320464>
} if self.auto_transitions:
for s in self.states.keys():
self.add_transition('to_%s' % s, self.wildcard_all, s) if trigger not in self.events:
self.events[trigger] = self._create_event(trigger, self)所以到这里我们的Machine实例字典参数如下：
self.events = {
'to_liquid': <Event('to_liquid')@4516320592>,
'to_gas': <Event('to_gas')@4516320912>,
'to_plasma': <Event('to_plasma')@4516321232>,
'to_solid': <Event('to_solid')@4516320528>
} if isinstance(source, string_types):
source = list(self.states.keys()) if source == self.wildcard_all else [source]
else:
source = [s.name if self._has_state(s) else s for s in listify(source)] for s in source:
d = s if dest == self.wildcard_same else dest
if self._has_state(d):
d = d.name
t = self._create_transition(s, d, conditions, unless, before,
after, prepare, **kwargs)
self.events[trigger].add_transition(t)所以到了这里我们的Machine实例参数如下:
self.events["to_solid"].transitions = {
'solid': [<Transition('solid', 'solid')@4516320656>],
'plasma': [<Transition('plasma', 'solid')@4516320848>],
'gas': [<Transition('gas', 'solid')@4516320784>],
'liquid': [<Transition('liquid', 'solid')@4516320720>]
}
self.events["to_liquid"].transitions = {
'solid': [<Transition('solid', 'liquid')@4516320656>],
'plasma': [<Transition('plasma', 'liquid')@4516320848>],
'gas': [<Transition('gas', 'liquid')@4516320784>],
'liquid': [<Transition('liquid', 'liquid')@4516320720>]
}
self.events["to_gas"].transitions = {
'solid': [<Transition('solid', 'gas')@4516320656>],
'plasma': [<Transition('plasma', 'gas')@4516320848>],
'gas': [<Transition('gas', 'gas')@4516320784>],
'liquid': [<Transition('liquid', 'gas')@4516320720>]
}
self.events["to_plasma"].transitions = {
'solid': [<Transition('solid', 'plasma')@4516320656>],
'plasma': [<Transition('plasma', 'plasma')@4516320848>],
'gas': [<Transition('gas', 'plasma')@4516320784>],
'liquid': [<Transition('liquid', 'plasma')@4516320720>]
}3、
Events
Machine的构造函数中，传入了transitions的list，触发了add_transitions()：
if transitions is not None:
self.add_transitions(transitions) for t in listify(transitions):
if isinstance(t, list):
self.add_transition(*t)self.events = {
'to_liquid': <Event('to_liquid')@4516320592>,
'to_gas': <Event('to_gas')@4516320912>,
'to_plasma': <Event('to_plasma')@4516321232>,
'to_solid': <Event('to_solid')@4516320528>，
'sublimate': <Event('sublimate')@4516446608>,
'evaporate': <Event('evaporate')@4516446480>,
'melt': <Event('melt')@4516446416>,
'ionize': <Event('ionize')@4516446736>
}self.events["melt"].transitions = {
'solid': [<Transition('solid', 'liquid')@4438964880>]
}
self.events["ionize"].transitions = {
'gas': [<Transition('gas', 'plasma')@4438965264>]
}
self.events["sublimate"].transitions = {
'liquid': [<Transition('liquid', 'gas')@4438965136>]
}
self.events["evaporate"].transitions = {
'solid': [<Transition('solid', 'gas')@4438965008>]
}4、
model
Machine的构造函数中，传入了model这list，触发了add_model()：
if model:
self.add_model(model) if hasattr(model, 'trigger'):
logger.warning("%sModel already contains an attribute 'trigger'. Skip method binding ", self.name)
else:
model.trigger = partial(get_trigger, model)实际上调用的是偏函数get_trigger(model, trigger_name).
def get_trigger(model, trigger_name, *args, **kwargs):
func = getattr(model, trigger_name, None)
if func:
return func(*args, **kwargs)
raise AttributeError("Model has no trigger named '%s'" % trigger_name)接着继续在add_model()中：
for trigger, _ in self.events.items():
self._add_trigger_to_model(trigger, model) def _add_trigger_to_model(self, trigger, model):
trig_func = partial(self.events[trigger].trigger, model)
setattr(model, trigger, trig_func)所以到了这里我们Machine实例关于model参数如下：
model.trigger("xxx")
model.to_liquid()
model.to_solid()
model.to_gas()
model.to_plasma()
model.melt()
model.mionize()
model.sublimate()
model.evaporate() for _, state in self.states.items():
self._add_model_to_state(state, model) def _add_model_to_state(self, state, model):
setattr(model, 'is_%s' % state.name,
partial(self.is_state, state.name, model))model.is_solid
model.is_liquid
model.is_gas
model.is_plasms
model.state # 这个是model.setState()把初始化状态如solid设置过来生成的属性好的，现在我们具体来看看调用model.melt()到底发生了什么呢？它其实是调用self.events['melt'].trigger(model)，
好的，看看self.events['melt'].trigger(model)里面是啥？
def trigger(self, model, *args, **kwargs):
f = partial(self._trigger, model, *args, **kwargs)
return self.machine._process(f) state = self.machine.get_state(model.state)
if state.name not in self.transitions:
msg = "%sCan't trigger event %s from state %s!" % (self.machine.name, self.name,
state.name)
if state.ignore_invalid_triggers:
logger.warning(msg)
return False
else:
raise MachineError(msg)然后判断state.name 在不在 当前event的transitions中：{'solid': [<Transition('solid', 'liquid')@4372347728>]}
好吧，确实存在，那么就不用if语句块啦。_trigger()继续往下：
event_data = EventData(state, self, self.machine, model, args=args, kwargs=kwargs) for t in self.transitions[state.name]:
event_data.transition = t
if t.execute(event_data):
event_data.result = True
break总结下，到这里我们执行model.melt()，其实就是执行self.events['melt'].transitions['solid'].execute().
好的，那我们继续看看这个t.execute(event_data)怎么个执行法。
因为我们没带什么prepare, conditions等高级参数，所以我们把这个execute()简化成：
def execute(self, event_data):
self._change_state(event_data)
return True def _change_state(self, event_data):
event_data.machine.get_state(self.source).exit(event_data)
event_data.machine.set_state(self.dest, event_data.model)
event_data.update(event_data.model)
event_data.machine.get_state(self.dest).enter(event_data)总结下，Machine就是有两个self.events={}, self.states=[]辅助围着self.model转，维护好这个model的状态，即可！
这个transitions库大量运用偏函数呀和动态生成属性函数什么的，不过这也正常，毕竟从demo上看来，demo越简单，底下的操作越复杂。
以上
